DK Eyewitness - Astronomy

Collenia
Fossilized algae
Dead plants and creatures buried in
sediment are slowly turned to rock,
becoming fossils. This rock contains the
fossilized remains of tiny algae that
were one of the earliest life forms.
HUMAN DAMAGE
Many scientists wonder if
humans, like the dinosaurs, might
also become extinct. The dinosaur
seems to have been a passive victim
of the changing Earth, while humans
are playing a key role in the
destruction of their environment.
In the year 2000 there were more
than 6 billion people on Earth—all
producing waste and pollution. In
addition to global warming that may
be occurring due to the greenhouse
effect, chemicals are being released
that deplete the ozone layer—a layer
in the atmosphere that keeps out
dangerous ultraviolet radiation.
Early life on earth
The first life on Earth was primitive plants that took carbon
dioxide from the air and released oxygen during photosynthesis.
Animals evolved when there was enough oxygen in the
atmosphere to sustain them. Knowledge about evolving life forms
comes in the form of fossils in the rocks (left). However, life
forms survive only if environmental conditions
on Earth are suitable for them.
The dinosaurs, for example,
though perfectly adapted
to their age, became
extinct about 65
million years ago.
Plant-eating
Edmontosaurus
Life-giving atmosphere
Our atmosphere extends out for about 600 miles (1,000 km).
It sustains life and protects us from the harmful effects of solar
radiation. It has several layers, but the life-sustaining layer is the
troposphere, up to 6 miles (10 km) above Earth’s surface.
4,500 mya
Carbon
dioxide
Hydrogen
Nitrogen
Oxygen
Today
Magnetosphere
shields Earth
from solar wind
Aurora
Meteor shower
(p.59)
Weather satellite
Ozone layer
Maximum height
for a balloon
Maximum height
for an airplane
Mount Everest
Troposphere
Clouds
Evolution of the atmosphere
Since Earth was formed, the chemical
makeup of the atmosphere has evolved.
Carbon dioxide (CO ² ) decreased significantly
between 4,500 and 3,000 million years
ago (mya). There was a comparable rise in
nitrogen. The levels of oxygen began to rise at
the same time, due to photosynthesis
of primitive plants, which used up
CO ² and released oxygen.
Facts about earth
THE SPHERICAL EARTH
As early as the 5th century bce
the Greek philosophers
had proposed that Earth is
spherical, and by the 3rd
century bce they had worked
out a series of experiments to
prove it. But it was not until the
first satellites were launched
in the late 1950s that humans
saw what their planet looks
like from space. The one
feature that makes Earth
unique is the great abundance
of liquid water; more than twothirds
of the surface is covered
with water. Water makes Earth
a dynamic place. Erosion, tides,
weather patterns, and plentiful
forms of life are all tied to the
presence of water. There is more
water in the Sahara Desert in North
Africa than there is on Venus (pp.46–47).
Oxygen/nitrogen
atmosphere
Solid iron core
Molten iron core
Rocky mantle
Rocky crust
• Sidereal period 365.26 days
• Temperature –95°F to 130°F (–70°C to 55°C)
• Rotational period 23 hr 56 min
• Mean distance from the Sun 93 million miles/
149.6 million km
• Volume 1 • Mass 1
• Density (water = 1) 5.52
• Equatorial diameter 7,930 miles/12,760 km
• Number of satellites 1 (the Moon)
43